Field of the Invention
This invention relates to a high frequency signal line adapted to be selectively used as an open-type (leakage) or as a closed-type (transmission) signal line.
Known high frequency signal lines, such as that described in Japanese Patent No. 2579583, typically include a coaxial cable having a central conductor and a cylindrical outer conductor. The outer conductor includes a longitudinal opening, and contains the central conductor therein. In addition, known signal lines of this type include a series of shielding covers arranged along the longitudinal opening of the outer conductor and adapted to be detachably attached to the opening.
The signal line also includes a directional coupling conductor for high frequency coupling. The directional coupling conductor is disposed within at least one of the shielding covers and is positioned to extend parallel to the central conductor. A matching resistor is connected to one end of the directional coupling conductor, and a branch line is connected to the other end of the directional coupling conductor. The space defined between the central conductor and the outer conductor is typically filled with an insulator.
As will be apparent from the foregoing description, known high frequency signal lines are constructed in a relatively simple manner in which the shielding cover is mounted outside of the opening of the outer conductor. However, such construction often makes it difficult to secure the shielding cover to the opening and inevitably is accompanied by a significant risk that the outer conductor might come in contact with the directional coupling conductor. In addition, with respect to high frequency signal lines, the strength of the branched output signal necessarily depends on the length of the induction line and, therefore, to adjust a strength of the output, known high frequency signal lines require induction lines of different lengths. Consequently, shielding covers of different lengths are also required.